In temperance tests (the equivalent of how many times your kid can poke your dog in the eye before it bites him) of all breeds the most tolerant was the Golden Retriever. The second most tolerant was the pit bull.
Pit Bull’s jaws do not lock, they do not have the most powerful bite among dogs (Rottweilers have that honor) they are not naturally human aggressive (in fact pit bull puppies prefer human company to their mother’s two weeks before all other dogs), and they feel as much pain as any other breed (accidentally step on one’s toe and you’ll see).
The most tolerant, patient, gentle breed of dogs is now embarrassingly portrayed as the most dangerous. It would be funny if the new reputation did not mean 6,000 are put to death every day, by far the highest number of any other breed euthanized.

The reason pit bulls have such a bad name is that they became famous among those who mistreat dogs for being a breed that is good for fighting, so those people bought many of them and mistreated them.

Environment > Genetics. Some of those dogs bit people and a reputation was born.

Malcolm Gladwell makes the most interesting point, which is that pit bulls are actually very aggressive--but only towards dogs--not humans.

It's stunning how wrong so many people can be about something with such a clear answer.